Device and method for compensating for phase distortion in base station of OFDMA-based cellular system

ABSTRACT

Disclosed is a device and method for compensating for phase distortions in a base station of an OFDMA-based cellular system. The method comprises receiving OFDM symbols from a plurality of mobile stations, canceling a symbol guard interval using a reference timing signal, and performing an FFT (fast Fourier transform) process on the OFDM symbols; dividing the OFDM symbols that have undergone FFT processing into subchannel groups of the mobile stations; restoring phases of the OFDM symbols divided into subchannel groups; and performing channel estimation and equalization on the restored OFDM symbols for each mobile station to thereby perform a demodulation process.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korea PatentApplication No. 2003-32932 filed on May 23, 2003 in the KoreanIntellectual Property Office, the content of which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a device and method for compensatingfor phase distortions in a base station of an OFDMA (orthogonalfrequency division multiple access) based cellular system. Morespecifically, the present invention relates to a device and method forcompensating for phase distortions in a base station of an OFDMA-basedcellular system, in which the device and method allow multiple access inthe uplink of the OFDMA-based cellular system.

(b) Description of the Related Art

Following developments in next-generation mobile communication systems,many methods have been proposed for providing various services includinghigh-quality and high-speed multimedia services. However, thedeterioration in performance caused by multipath fading channels inmobile communication environments has become serious impediments torealizing such services.

Therefore, many techniques for overcoming the deterioration inperformance caused by multipath fading have been developed and used. Adrawback of these techniques, however, is that although they minimizesuch performance deterioration, the techniques require the design of acomplex receiver.

The OFDMA method has been suggested for easily solving the problem ofdeterioration in performance caused by multipath fading by using asimple demodulator.

In the OFDMA method, a total of N subcarriers are divided into groups ina single OFDM symbol and in such a manner that the subcarriers are notrepeated, and one of the divided groups (or subchannels) is allocated toeach mobile station user.

In the OFDMA method, each mobile station loads data to a subcarrier inthe group and transmits the data during a predetermined time frame inthe reverse link case. Since the subcarriers included in the subchannelallocated to users who stay in a single cell belong to a single OFDMAsymbol, it is necessary for a base station receiver to performsynchronization between each of the subchannels received from the mobilestation. That is, in order to perform accurate demodulation, it isnecessary that the base station receiver perform the same FFT (fastFourier transform) at the same symbol timing.

Hence, OFDMA flexibly processes various services for the multitude ofrequests made by users, but results in the deterioration of performancecompared to other multi-user access methods when the multiple users arenot synchronized in the uplink.

SUMMARY OF THE INVENTION

It is an advantage of the present invention to provide a device andmethod for compensating for phase distortions in a base station of anOFDMA-based cellular system, in which the device and method simply andstably demodulate data of mobile station users at a base stationreceiver without performing an additional synchronization processbetween the mobile station users in the reverse link.

In one aspect of the present invention, a method for compensating forphase distortions in a base station of an OFDMA-based cellular systemcomprises (a) receiving OFDM symbols from a plurality of mobilestations, canceling a symbol guard interval using a reference timingsignal, and performing an FFT (fast Fourier transform) process on theOFDM symbols; (b) dividing the OFDM symbols that have undergone FFTprocessing into subchannel groups of the mobile stations; (c) restoringphases of the OFDM symbols divided into subchannel groups; and (d)performing channel estimation and equalization on the restored OFDMsymbols for each mobile station to thereby perform a demodulationprocess.

In another aspect of the present invention, in a device for compensatingfor phase distortions of OFDM symbols received from a plurality ofmobile stations in a base station of an OFDMA based cellular system, aphase distortion compensator in the base station of the OFDMA basedcellular system comprises a symbol guard interval canceller forcanceling a symbol guard interval of the OFDM symbols of the pluralityof mobile stations received at the base station; an FFT (fast Fouriertransform) processor for performing an FFT process on the OFDM symbolswith the cancelled symbol guard interval; a subchannel divider forextracting subchannels allocated to each mobile station from the OFDMsymbols that have undergone the FFT process; a symbol timing estimatorfor estimating a time delay between a timing of each OFDM symbolreceived from the mobile station and a reference symbol timing of thebase station; a delay time phase compensator for compensating for phasedistortions of the OFDM symbols of the mobile stations of thesubchannels extracted by the subchannel group divider by using the delaytime estimated by the symbol timing estimator; and a channel estimationand equalizer for performing distortion correction of the OFDM symbolsof the mobile stations of the subchannels compensated by the delay timephase compensator, the distortion correction being performed accordingto an amplitude and a phase resulting from a signal channel of themobile station.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate an embodiment of the invention,and, together with the description, serve to explain the principles ofthe invention:

FIG. 1 shows a block diagram of a reverse link in an OFDMA-basedcellular system;

FIG. 2 shows transmit timing signals received at a base stationconnected to mobile stations of FIG. 1, and shows also transmit timingsignals of the base station;

FIG. 3 shows an OFDM symbol configuration, and a relation between atransmit timing signal of a base station and a transmit timing signal ofa mobile station;

FIG. 4 shows a block diagram of a phase distortion compensator in a basestation of an OFDMA-based cellular system according to a preferredembodiment of the present invention; and

FIG. 5 shows an operational flowchart for a phase distortioncompensation method in a base station of an OFDMA-based cellular systemaccording to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, only the preferred embodiment ofthe invention has been shown and described, simply by way ofillustration of the best mode contemplated by the inventor(s) ofcarrying out the invention. As will be realized, the invention iscapable of modification in various obvious respects, all withoutdeparting from the invention. Accordingly, the drawings and descriptionare to be regarded as illustrative in nature, and not restrictive.

FIG. 1 shows a block diagram of a reverse link in an OFDMA-basedcellular system.

Referring to FIG. 1, n mobile stations 200-1 through 200-n in anOFDMA-based cellular system are respectively required to have delaytimes of d₁, d₂, . . . , d_(n) with respect to a transmit symbol timingof the base station 100 in order to transmit data to the base station100 through a reverse link.

The transmit timing of the mobile stations 200-1 through 200-n and ofthe base station 100 have the relation as described below.

FIG. 2 shows transmit timing signals received at a base stationconnected to mobile stations of FIG. 1, and shows also transmit timingsignals of the base station.

Referring to FIG. 2, if it is assumed that a receiver of the basestation 100 has acquired a symbol timing of the first mobile station200-1, symbol timing errors between the mobile stations 200-2 through200-n and the mobile station 200-1 are generated even though thereceiver of the base station 100 synchronizes its own timing withrespect to the symbol timing received from the first mobile station200-1. Therefore, the data of other mobile stations are distorted, andoverall performance deterioration occurs because of the distortion.

The degree of interference is described below.

FIG. 3 shows an OFDM symbol configuration, and a relation between atransmit timing signal of a base station and a transmit timing signal ofa mobile station.

In particular, FIG. 3 shows an OFDM symbol configuration between areceiver of the base station 100 and the first mobile station 200-1 thatinitially acquires the symbol timing.

The receiver of the base station 100 accurately estimates a startingpoint (a) of the OFDM symbol of the first mobile station 200-1 so as tosuccessfully demodulate a signal of the first mobile station 200-1, andadds a pre-established symbol guard interval (b) (referred to as a CP,or cyclic prefix hereinafter) to the estimated starting point (a) tofind an FFT starting point (c).

When the receiver of the base station 100 performs an FFT process on thedata in an FFT interval (d) beginning from the found FFT starting point(c) to extract data allocated to a subchannel of the corresponding firstmobile station 200-1, the result is that the signal of the first mobilestation 200-1 is successfully demodulated.

If, instead of using the symbol timing of the first mobile station200-1, the reference timing {circle around (1)} of the base station 100is regarded as a symbol sync of the first mobile station 200-1 todemodulate the same, the characteristics of distortions caused by timingerrors are altered according to the delay time d₁ of the first mobilestation 200-1. When the symbol timing of the first mobile station 200-1is actually provided in the CP ({circle around (2)} of FIG. 3), it isinfluenced by Equation 1.Y_(l,k)=X_(l,k)H_(l,k)e^(−j2πkd) ¹ ^(/N)+W_(l,k)   Equation 1

where Y_(l,k) is a demodulation signal, H_(l,k) is a transmit signal,W_(l,k) is a multipath fading channel of the k-th sub-carrier of the1-th OFDM symbol in the frequency domain, and N is a size of the FFT.

When the symbol timing of the mobile station is outside the CP {circlearound (2)}, phase distortion, and subcarrier interference and ISI(inter symbol interference) allocated to other mobile stations aregenerated at the demodulated data as given in Equation 2.Y_(l,k)=e^(j2πεk/N)α(ε)X_(l,k)H_(l,k)+n_(l,kε)+W_(l,k)   Equation 2

where ε is a relative delay time between the base station referencetiming and the mobile station, n_(l,k,ε) is an interference between theISI and inter subcarrier interference, and

${\alpha(ɛ)} = {\sum\limits_{i}\;\left| {h_{i}(t)} \middle| {}_{2}\frac{N - ɛ_{i}}{N} \right.}$is an attenuation term of a symbol. Therefore, symbol timing errors ofeach of the mobile stations 200-1 through 200-n may occur in the CP{circle around (2)} or the FFT intervals {circle around (4)} and {circlearound (5)}, and accordingly, different distortions occur.

An OFDMA-based cellular system for preventing distortions will now bedescribed.

FIG. 4 shows a block diagram of a phase distortion compensator in a basestation of an OFDMA-based cellular system according to a preferredembodiment of the present invention.

Referring to FIG. 4, the phase distortion compensator 400 of the basestation 100 of the OFDMA-based cellular system comprises a CP canceller410, an FFT processor 420, a subchannel divider 430, delay time phasecompensators 440-1 through 440-n, channel estimation and equalizers450-1 through 450-n, and a symbol timing estimator 460. The phasedistortion compensator 400 is a device provided in the receiver of thebase station 100, with a plurality of phase distortion compensators 400being provided therein for each of the mobile stations 200-1 through200-n. For illustration purposes, only one of the phase distortioncompensators 400 is shown in the drawing.

The symbol timing estimator 460 comprises a timing controller 461 and atiming offset estimator 462.

The phase distortion compensator 400 performs phase distortioncompensation of the OFDM symbol received from the mobile stations 200-1through 200-n. The receiver of the base station 100 processes the signalcompensated by the phase distortion compensator 400.

The CP canceller 410 cancels a CP (b) of the OFDM symbol provided fromall the mobile stations 200-1 through 200-n and received at the basestation 100, and the FFT processor 420 demodulates the CP-cancelled OFDMsymbol for all the subchannels.

The subchannel divider 430 divides the signal demodulated by the FFTprocessor 420 into respective mobile station data by using a specifictone which is used by each mobile station 200-1 through 200-n for datamodulation.

The timing offset estimator 462 of the symbol timing estimator 460estimates delay times of base station transmit symbol timings of themobile stations 200-1 through 200-n, and groups together the mobilestations with distributed delay times for a predetermined duration oftime. The timing controller 461 generates a first reference time R₁ byusing a symbol timing of the mobile station with the shortest delay inthe group, and converts delay times of the mobile stations belonging tothe group into relative delay times ε₁ through ε_(n) by using thereference time R₁ in the calculation of the relative delay times ε₁through ε_(n). That is, the relative delay times ε₁ through ε_(n) arefound using Equation 3.ε_(i)=d_(i)−R₁   Equation 3

where ε_(i) is a relative delay time of the i-th mobile station andd_(i) is a time delay of the i-th mobile station.

The phase distortion compensator 400 demodulates a signal received fromthe mobile stations 200-1 through 200-n based on the base stationreference time R₁. Distortion of the demodulation data caused by thesymbol timing errors of the mobile stations 200-1 through 200-nbelonging to each of the groups generates phase errors as given inEquation 1. This will now be described in detail.

Referring to FIG. 3, the symbol starting point (a) of the first mobilestation 200-1 is delayed by ε₁ beginning at the base station OFDM symbolstart {circle around (1)}. Therefore, when the base station referencetime R₁ is established based on the mobile station with the shortestdelay time of the first group, the timing errors of all the mobilestations belonging to the corresponding group are generated in the CP.Accordingly, signals of the mobile stations 200-1 through 200-n have aphase distortion corresponding only to a difference between the basestation transmit symbol timing and the symbol timings of the mobilestations 200-1 through 200n as evident from Equation 1. The result ofthis is that the phase distortion on the respective subchannelsbelonging to the mobile stations 200-1 through 200-n can be accuratelyrestored if the relative delay times ε₁ through ε_(n) can be accuratelyestimated.

The symbol timing estimator 460 performs grouping of all the mobilestations 200-1 through 200-n within a cell as described above, andgenerates a base station reference timing and a relative delay time forthe phase distortion compensator 400 so that the phase distortioncompensator 400 may perform accurate phase distortion compensation.

In addition, the symbol timing estimator 460 estimates relative delaytimes of each of the mobile stations 200-1 through 200-n by applyingspecific modulated codes to preambles of the mobile stations 200-1through 200-n.

The delay time phase compensators 440-1 through 440-n restore adistorted phase of the data of the corresponding mobile stationaccording to the relative delay time estimated by the symbol timingestimator 460. Restoration is performed as shown in Equation 4 by adegree corresponding to the level of phase distortion indicated inEquation 1. Equation 4{circumflex over (X)}_(l,k)=Y_(l,k)e^(j2πkδ) ¹ ^(/N)

The channel estimation and equalizers 450-1 through 450-n performchannel estimation on the restored data signals of each of the mobilestations 200-1 through 200-n, and equalizes channel-estimated signals tothereby normally process the signals.

According to the above-described method, since the receiver of the basestation requires no additional synchronization process between themobile stations 200-1 through 200-n by the phase distortion compensator400 and dynamically operates according to delay time distributions ofthe mobile stations within the cell, the design of the receiver need notbe complicated.

A sync distortion compensation method in the OFDMA-based cellular systemaccording to the preferred embodiment of the present invention will nowbe described.

FIG. 5 shows an operational flowchart for a phase distortioncompensation method in a base station of the OFDMA-based cellular systemaccording to a preferred embodiment of the present invention.

Referring to FIG. 5, the base station 100 transmits a signal accordingto an established transmit symbol timing in step S501. The mobilestations 200-1 through 200-n then acquire transmit symbol timingsdelayed by a predetermined time from the base station 100, and transmitdata to the receiver of the base station 100 by applying each of thetransmit symbol timings through the reverse link in step S502. The OFDMsymbols from the mobile stations 200-1 through 200-n received at thereceiver of the base station 100 have their phase distortion compensatedfor through the phase distortion compensator 400 of the receiver of thebase station 100, and the receiver of the base station 100 processesresidual data of the signal that has undergone phase distortioncompensation.

The symbol timing estimator 460 of the base station 100 estimates adelay time of each mobile station, groups together the mobile stationsthat are provided within a given time interval, and calculates areference timing signal for each group and a relative delay with respectto the reference timing in step S503.

The CP controller 410 of the phase distortion compensator 400 cancels aCP of the OFDM symbol received from the mobile stations 200-1 through200-n according to the reference timing signal of the base station 100obtained in step S503, and the FFT processor 420 performs an FFT processon the CP-cancelled OFDM symbol from the mobile stations 200-1 through200-n in step S504. Therefore, without performing any precise control ofthe symbol timing of the mobile stations 200-1 through 200-n, the phasedistortion compensator 400 is able to perform FFT processes on the OFDMsymbol received through the reverse link from the mobile stations 200-1through 200-n according to a receive symbol timing of the base station100.

The subchannel divider 430 divides the OFDM symbol that has undergoneFFT processing in step S504 into subchannel groups of the mobilestations 200-1 through 200-n in step S505. Next, in step S506, thesymbol timing estimator 460 measures delay times of the mobile stations200-1 through 200-n, and the delay time phase compensators 440-1 through440-n compensate for the phases of the signals of the mobile stations200-1 through 200-n by using the delay time estimated by the symboltiming estimator 460.

The channel estimation and equalizers 450-1 through 450-n perform anadditional demodulation process on the phase-compensated signals of themobile stations 200-1 through 200-n to thereby complete phase distortioncompensation in step S507.

In the additional demodulation process, channel estimation andequalization are performed to reduce residual distortion by adding apilot for channel estimation of the subchannel groups of each of themobile stations 200-1 through 200-n.

As described above, the phase distortion compensation device and methodin a base station of the OFDMA-based cellular system requires noadditional synchronization between mobile stations in a multi-userenvironment and in the reverse link of the OFDMA, and demodulates thedata between multi-users without the use of a complicated design of thebase station receiver. Further, the present invention can be applied toa system for synchronization between the mobile stations.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment, but, on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method for compensating for phase distortions in a base station ofan OFDMA (orthogonal frequency division multiple access) based cellularsystem, comprising: (a) receiving OFDM (orthogonal frequency divisionmultiplexing) symbols from a plurality of mobile stations; (b) groupingthe plurality of the mobile stations according to a predeterminedduration of time, and generating a reference timing signal for eachgroup and relative delay times among the mobile stations; (c) cancelinga symbol guard interval using the reference timing signal, andperforming an FFT (fast Fourier transform) process on the OFDM symbols;(d) dividing the OFDM symbols that have undergone FFT processing intosubchannel groups of the mobile stations; (d)(e) restoring phases of theOFDM symbols divided into subchannel groups based on the estimation ofthe relative delay times among the mobile stations; and (e)(f)performing channel estimation and equalization on the restored OFDMsymbols for each mobile station to thereby perform a demodulationprocess.
 2. The method of claim 1, wherein as to the reference timingsignal, predetermined mobile stations with delay times shorter than apredetermined time are formed into the group, and the reference timingsignal for decoding mobile station signals of this group is generated.3. The method of claim 2, wherein the reference timing signal isobtained based on the delay time of one of the mobile stations with theshortest delay time within the group.
 4. The method of claim 1, whereinthe FFT process in (a) is performed according to a reference symboltiming of the base station.
 5. The method of claim 1, wherein in (e),the phase distorted OFDM symbols of the mobile stations are restored bythe relative delay times calculated based on a difference between adelay time of the base station and a reference time resulting from thereference timing signal.
 6. The method of claim 1, wherein (f) comprisesperforming channel estimation and equalization to reduce residualdistortions.
 7. In a device for compensating for phase distortion ofOFDM symbols received from a plurality of mobile stations in a basestation of an OFDMA (orthogonal frequency division multiple access)based cellular system, a phase distortion compensator in the basestation of the OFDMA-based cellular system, comprising: a symbol guardinterval canceller for canceling a symbol guard interval of the OFDMsymbols of the plurality of mobile stations received at the basestation, the symbol guard interval canceller to cancel the symbol guardtime using a reference time; an FFT (fast Fourier transform) processorfor performing an FFT process on the OFDM symbols with the cancelledsymbol guard interval; a subchannel divider for extracting subchannelsallocated to each mobile station from the OFDM symbols that haveundergone the FFT process; a symbol timing estimator for grouping theplurality of the mobile stations according to a predetermined durationof time, and generating the reference timing signal for each group andestimating the relative delay times among the OFDM symbols received fromthe mobile stations; a delay time phase compensator for compensating forphase distortions of the OFDM symbols of the mobile stations of thesubchannels extracted by the subchannel group divider by using therelative delay times estimated by the symbol timing estimator; and achannel estimation and equalizer for performing distortion correction ofthe OFDM symbols of the mobile stations of the subchannels compensatedby the delay time phase compensator, the distortion correction beingperformed according to an amplitude and a phase resulting from a signalchannel of the mobile station.
 8. The device of claim 7, wherein thesymbol timing estimator further comprises: a timing offset estimator forestimating delay times of the mobile stations with respect to a transmitsymbol timing of the base station; and a timing controller for groupingtogether the mobile stations according to the delay times of the mobilestations estimated by the timing offset estimator, obtaining thereference time using a symbol timing of the mobile station with theshortest delay time in each group, and obtaining the relative delaytimes by using the reference time.
 9. A method for compensating forphase distortions in a base station of an orthogonal frequency divisionmultiple access based cellular system, comprising: receiving orthogonalfrequency division multiplexing symbols from a plurality of mobilestations having a respective delay time; grouping the plurality of themobile stations according to a predetermined duration of time, andgenerating a reference timing signal for each group and relative delaytimes of the mobile stations; removing a symbol guard interval using thereference timing signal, and processing the orthogonal frequencydivision multiplexing symbols; and grouping the orthogonal frequencydivision multiplexing symbols that have undergone processing intosubchannel groups of the mobile stations.
 10. The method of claim 9,further comprising: restoring phases of the orthogonal frequencydivision multiplexing symbols grouped into subchannel groups based onthe estimation of the relative delay times among the mobile stations;and performing channel estimation and equalization on the restoredorthogonal frequency division multiplexing symbols for each mobilestation to thereby perform a demodulation process.
 11. The method ofclaim 10, wherein as to the reference timing signal, predeterminedmobile stations with delay times shorter than a predetermined time areformed into the group, and the reference timing signal for decodingmobile station signals of this group is generated.
 12. The method ofclaim 11, wherein the reference timing signal is obtained based on thedelay time of one of the mobile stations with the shortest delay timewithin the group.
 13. A phase distortion compensator in the base stationof an orthogonal frequency division multiple access based cellularsystem, comprising: a symbol guard interval canceller for cancelingsymbol guard intervals of orthogonal frequency division multiplexingsymbols of a plurality of mobile stations received at the base station,the symbol guard interval canceller to cancel the symbol guard intervalsusing a reference time; a processor for processing the orthogonalfrequency division multiplexing symbols; a subchannel grouper forextracting subchannels allocated to each mobile station from theorthogonal frequency division multiplexing symbols that have undergoneprocessing; and a symbol timing estimator for grouping the plurality ofthe mobile stations according to a predetermined duration of time, andgenerating the reference timing signal for each group and estimating therelative delay times of the orthogonal frequency division multiplexingsymbols received from the mobile stations.
 14. The phase distortioncompensator of claim 13, further comprising: a delay time phasecompensator for compensating for phase distortions of the orthogonalfrequency division multiplexing symbols of the mobile stations of thesubchannels extracted by the subchannel grouper by using the relativedelay times estimated by the symbol timing estimator.
 15. The phasedistortion compensator of claim 14, further comprising: a channelestimation and equalizer for performing distortion correction of theorthogonal frequency division multiplexing symbols of the mobilestations of the subchannels compensated by the delay time phasecompensator, the distortion correction being performed according to anamplitude and a phase resulting from a signal channel of the mobilestation.
 16. The phase distortion compensator of claim 15, wherein as tothe reference timing signal, predetermined mobile stations with delaytimes shorter than a predetermined time are formed into the group, andthe reference timing signal for decoding mobile station signals of thisgroup is generated.
 17. The phase distortion compensator of claim 16,wherein the reference timing signal is obtained based on the delay timeof one of the mobile stations with the shortest delay time within thegroup.